Bone is a composite material composed of impure hydroxyapatite, collagen, and a variety of non-collagenous proteins, as well as embedded and adherent cells. Bone can be processed into an implantable biomaterial, such as an allograft, for example, by removing the cells, leaving behind the extracellular matrix. The processed bone material can have a variety of properties, depending upon the specific processes and treatments applied to it, and may incorporate characteristics of other biomaterials with which it is combined. For example, bone-derived biomaterials may be processed into load-bearing mineralized grafts that support and integrate with the patient's bone and may alternatively be processed into soft, moldable, or flowable demineralized bone materials that have the ability to induce a cellular healing response.
The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which the bone is unable to support physiologic loading. Metal pins, screws, and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Furthermore, most metal implants are permanent and unable to participate in physiological remodeling.
Bone's cellular healing processes, using bone tissue formation by osteoblast cells coordinated with bone and graft resorption by osteoclast cells, permit bone grafts and certain bone substitute materials to remodel into endogenous bone that is almost indistinguishable from the original. However, the use of bone grafts is limited by the available shape and size of grafts and the desire to optimize both mechanical strength and degradation rate. Variations in bone size and shape among patients (and donors) also make bone grafts a less optimal substitute material. Bone substitute materials and bone chips are quickly remodeled but cannot immediately provide mechanical support. In contrast, cortical bone grafts can support physiological stresses but remodel slowly.
Methods have been developed for preparing composites (see, for example, U.S. Pat. Nos. 5,507,813; 5,899,939; 6,123,731; 6,294,041; 6,294,187; 6,332,779; 6,440,444; and 6,478,825, each of which is incorporated herein by reference) including allogenic bone for use in load bearing orthopedic applications. However, in some applications, it is desirable to increase the rate at which native tissue penetrates implanted material, while it may not be necessary that the material actually bear weight. In these applications, it is desirable to have an implantable material that is optimized for infiltration with less emphasis on mechanical strength.